package/mupdf: fix building with mips toolchains

With some toolchains (e.g. mips64el), partial linking fails in the
following way:
/tmp/instance-1/output-1/host/opt/ext-toolchain/bin/../lib/gcc/mips64el-buildroot-linux-uclibc/8.4.0/../../../../mips64el-buildroot-linux-uclibc/bin/ld: build/release/libmupdf.a(Dingbats.cff.o): ABI is incompatible with that of the selected emulation
/tmp/instance-1/output-1/host/opt/ext-toolchain/bin/../lib/gcc/mips64el-buildroot-linux-uclibc/8.4.0/../../../../mips64el-buildroot-linux-uclibc/bin/ld: failed to merge target specific data of file build/release/libmupdf.a(Dingbats.cff.o)

Taking inspiration from commit
9eca4b9f, fix it by using GCC instead
of LD for partial linking.

Note that on mips the build will now produce warnings similar to this
one:
buildroot/output/host/lib/gcc/mips64el-buildroot-linux-gnu/10.3.0/../../../../mips64el-buildroot-linux-gnu/bin/ld: build/release/libmupdf.a(NotoSansTaiTham-Regular.ttf.o): warning: linking abicalls files with non-abicalls files

During a runtime test on mips64el under qemu, mupdf-x11 was
nonetheless able to display a sample PDF file correctly.

Fixes:
- http://autobuild.buildroot.net/results/156fe9ee5f6dccdc98990f6c5de5562383bc2b74/



Signed-off-by: Raphaël Mélotte <raphael.melotte@mind.be>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>