Forwarded from https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99092 when building GCC with LLVM's assembler.
But can be reproduced with an inline-asm as simple as:

void f(void)
{
        asm("prfm    PLDL1KEEP, [x0, #-8]");
}

My analysis of why this is a LLVM assembler issue can be found at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99092#c9 .
The ARM folks specifically wanted to output pfrm from GCC too:
https://gcc.gnu.org/legacy-ml/gcc-patches/2014-07/msg00612.html

When it comes to emitting the pattern, always use "prfm" -- the prfum
form can be generated from the prfm mnemonic when the offset implies
this is necessary.

So I suspect this should be accepted by the LLVM assembler also.