[AArch64] Rename +bitperm to +sve2-bitperm

[AArch64] Rename +bitperm to +sve2-bitperm

After some discussion, we've decided to rename the +bitperm feature
flag to +sve2-bitperm, so that it's consistent with the other SVE2
feature flags.  The associated macro was already
__ARM_FEATURE_SVE2_BITPERM, so only the feature flag itself
needs to change.

2019-07-19  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* doc/invoke.texi: Rename the AArch64 +bitperm extension flag
	to +sve-bitperm.
	* config/aarch64/aarch64-option-extensions.def: Likewise.

From-SVN: r273600