build: make sure asm gets built with -DPIC

build: make sure asm gets built with -DPIC

Fixes issue openwrt/packages#14921, whereby inline ASM wasn't getting
built as PIC; look at gmp-6.2.1/mpn/x86/pentium/popcount.asm for
example:

ifdef(`PIC',`
...

for a routine that exists in both PIC and non-PIC versions.

Make sure that wherever $(FPIC) gets passed as a variable expansion
that it gets quoted where necessary (such as setting environment
variables in shell commands).

Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>
(cherry picked from commit af22991e03cae55f96b06996df2ff16752cec5d5)