devel/llvm90: enable COMPILER_RT on riscv64

1. COMPILER_RT builds libclang_rt.builtins-riscv64.a library, but not cfi_blacklist.txt, so _COMPILER_RT_BLACKLISTS needs to be put into amd64 and i386 cases.
2. Instead of:

.if ${ARCH}
.endif
.if {ARCH}
Do:
.if ${ARCH}
.elseif ${ARCH}

3. Remove whitespace at the end of line.

PR: 261106
Approved by: brooks (maintainer)