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
