[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 internal macros already used
"SVE2_BITPERM", so only the feature flag itself needs to change.

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

gas/
	* doc/c-aarch64.texi: Remame the +bitperm extension to +sve2-bitperm.
	* config/tc-aarch64.c (aarch64_features): Likewise.
	* testsuite/gas/aarch64/illegal-sve2-aes.d: Update accordingly.
	* testsuite/gas/aarch64/illegal-sve2-sha3.d: Likewise.
	* testsuite/gas/aarch64/illegal-sve2-sm4.d: Likewise.
	* testsuite/gas/aarch64/illegal-sve2.d: Likewise.
	* testsuite/gas/aarch64/sve2.d: Likewise.