[AsmParser] Make generic directives and aliases case insensitive.

GCC will accept any case for assembler directives.
For example ".abort" and ".ABORT" (even ".aBoRt")
are equivalent.

https://sourceware.org/binutils/docs/as/Pseudo-Ops.html#Pseudo-Ops
"The names are case insensitive for most targets,
and usually written in lower case."

Change llvm-mc to accept any case for generic directives
or aliases of those directives.

This for Bugzilla #39527.

Differential Revision: https://reviews.llvm.org/D72686