[libc] Dispatch memmove to memcpy when buffers are disjoint
Most of the time `memmove` is called on buffers that are disjoint, in that case we can use `memcpy` which is faster.
The additional test is branchless on x86, aarch64 and RISCV with the zbb extension (bitmanip).
On x86 this patch adds a latency of 2 to 3 cycles.
Before
```
--------------------------------------------------------------------------------
Benchmark Time CPU Iterations UserCounters...
--------------------------------------------------------------------------------
BM_Memmove/0/0_median 5.00 ns 5.00 ns 10 bytes_per_cycle=1.25477/s bytes_per_second=2.62933G/s items_per_second=199.87M/s __llvm_libc::memmove,memmove Google A
BM_Memmove/1/0_median 6.21 ns 6.21 ns 10 bytes_per_cycle=3.22173/s bytes_per_second=6.75106G/s items_per_second=160.955M/s __llvm_libc::memmove,memmove Google B
BM_Memmove/2/0_median 8.09 ns 8.09 ns 10 bytes_per_cycle=5.31462/s bytes_per_second=11.1366G/s items_per_second=123.603M/s __llvm_libc::memmove,memmove Google D
BM_Memmove/3/0_median 5.95 ns 5.95 ns 10 bytes_per_cycle=2.71865/s bytes_per_second=5.69687G/s items_per_second=167.967M/s __llvm_libc::memmove,memmove Google L
BM_Memmove/4/0_median 5.63 ns 5.63 ns 10 bytes_per_cycle=2.28294/s bytes_per_second=4.78383G/s items_per_second=177.615M/s __llvm_libc::memmove,memmove Google M
BM_Memmove/5/0_median 5.68 ns 5.68 ns 10 bytes_per_cycle=2.16798/s bytes_per_second=4.54295G/s items_per_second=176.015M/s __llvm_libc::memmove,memmove Google Q
BM_Memmove/6/0_median 7.46 ns 7.46 ns 10 bytes_per_cycle=3.97619/s bytes_per_second=8.332G/s items_per_second=134.044M/s __llvm_libc::memmove,memmove Google S
BM_Memmove/7/0_median 5.40 ns 5.40 ns 10 bytes_per_cycle=1.79695/s bytes_per_second=3.76546G/s items_per_second=185.211M/s __llvm_libc::memmove,memmove Google U
BM_Memmove/8/0_median 5.62 ns 5.62 ns 10 bytes_per_cycle=3.18747/s bytes_per_second=6.67927G/s items_per_second=177.983M/s __llvm_libc::memmove,memmove Google W
BM_Memmove/9/0_median 101 ns 101 ns 10 bytes_per_cycle=9.77359/s bytes_per_second=20.4803G/s items_per_second=9.9333M/s __llvm_libc::memmove,uniform 384 to 4096
```
After
```
BM_Memmove/0/0_median 3.57 ns 3.57 ns 10 bytes_per_cycle=1.71375/s bytes_per_second=3.59112G/s items_per_second=280.411M/s __llvm_libc::memmove,memmove Google A
BM_Memmove/1/0_median 4.52 ns 4.52 ns 10 bytes_per_cycle=4.47557/s bytes_per_second=9.37843G/s items_per_second=221.427M/s __llvm_libc::memmove,memmove Google B
BM_Memmove/2/0_median 5.70 ns 5.70 ns 10 bytes_per_cycle=7.37396/s bytes_per_second=15.4519G/s items_per_second=175.399M/s __llvm_libc::memmove,memmove Google D
BM_Memmove/3/0_median 4.47 ns 4.47 ns 10 bytes_per_cycle=3.4148/s bytes_per_second=7.15563G/s items_per_second=223.743M/s __llvm_libc::memmove,memmove Google L
BM_Memmove/4/0_median 4.53 ns 4.53 ns 10 bytes_per_cycle=2.86071/s bytes_per_second=5.99454G/s items_per_second=220.69M/s __llvm_libc::memmove,memmove Google M
BM_Memmove/5/0_median 4.19 ns 4.19 ns 10 bytes_per_cycle=2.5484/s bytes_per_second=5.3401G/s items_per_second=238.924M/s __llvm_libc::memmove,memmove Google Q
BM_Memmove/6/0_median 5.02 ns 5.02 ns 10 bytes_per_cycle=5.94164/s bytes_per_second=12.4505G/s items_per_second=199.14M/s __llvm_libc::memmove,memmove Google S
BM_Memmove/7/0_median 4.03 ns 4.03 ns 10 bytes_per_cycle=2.47028/s bytes_per_second=5.17641G/s items_per_second=247.906M/s __llvm_libc::memmove,memmove Google U
BM_Memmove/8/0_median 4.70 ns 4.70 ns 10 bytes_per_cycle=3.84975/s bytes_per_second=8.06706G/s items_per_second=212.72M/s __llvm_libc::memmove,memmove Google W
BM_Memmove/9/0_median 90.7 ns 90.7 ns 10 bytes_per_cycle=10.8681/s bytes_per_second=22.7739G/s items_per_second=11.02M/s __llvm_libc::memmove,uniform 384 to 4096
```
Reviewed By: courbet
Differential Revision: https://reviews.llvm.org/D152811