hexagon: work around compiler crash

Clang cannot translate the string "r30" into a valid register yet.

Link: https://github.com/ClangBuiltLinux/linux/issues/755
Link: http://lkml.kernel.org/r/20191028155722.23419-1-ndesaulniers@google.com


Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Suggested-by: Sid Manning <sidneym@quicinc.com>
Reviewed-by: Brian Cain <bcain@codeaurora.org>
Cc: Allison Randal <allison@lohutok.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Richard Fontana <rfontana@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>