arm: Fix CMSE support detection in libgcc (PR target/99157)

arm: Fix CMSE support detection in libgcc (PR target/99157)

As discussed in the PR, the Makefile fragment lacks a double '$' to
get the return-code from GCC invocation, resulting is CMSE support
missing from multilibs.

I checked that the simple patch proposed in the PR fixes the problem.

2021-02-23  Christophe Lyon  <christophe.lyon@linaro.org>
	    Hau Hsu  <hsuhau617@gmail.com>

	PR target/99157
	libgcc/
	* config/arm/t-arm: Fix cmse support detection.

(cherry picked from commit be30dd89926d5dd19d72f90c1586b0e2557fde43)