mips: Do not include hi and lo in __SYSCALL_CLOBBERS for R6

mips: Do not include hi and lo in __SYSCALL_CLOBBERS for R6

GCC 10 (PR 91233) won't silently allow registers that are not architecturally
available to be present in the clobber list anymore, resulting in build failure
for mips*r6 targets in form of:
...
.../sysdep.h:146:2: error: the register ‘lo’ cannot be clobbered in ‘asm’ for the current target
  146 |  __asm__ volatile (      \
      |  ^~~~~~~

This is because base R6 ISA doesn't define hi and lo registers w/o DSP extension.
This patch provides the alternative definitions of __SYSCALL_CLOBBERS for r6
targets that won't include those registers.

	* sysdeps/unix/sysv/linux/mips/mips32/sysdep.h (__SYSCALL_CLOBBERS): Exclude
	hi and lo from the clobber list for __mips_isa_rev >= 6.
	* sysdeps/unix/sysv/linux/mips/mips64/n32/sysdep.h (__SYSCALL_CLOBBERS): Likewise.
	* sysdeps/unix/sysv/linux/mips/mips64/n64/sysdep.h (__SYSCALL_CLOBBERS): Likewise.