gnu: libgcrypt: Fix cross-compilation.

gnu: libgcrypt: Fix cross-compilation.

This works around

    /gnu/store/pwcp239kjf7lnj5i4lkdzcfcxwcfyk72-bash-minimal-5.0.16/bin/bash ../libtool  --tag=CC   --mode=link i586-pc-gnu-gcc -I/gnu/store/8x9dhckldm6iy1digklfanmvdaizq11f-libgpg-error-1.37/include -g -O2 -fvisibility=hidden -fno-delete-null-pointer-checks -Wall   -o mpicalc mpicalc-mpicalc.o libgcrypt.la  -L/gnu/store/8x9dhckldm6iy1digklfanmvdaizq11f-libgpg-error-1.37/lib -lgpg-error

    i586-pc-gnu-ld: ./.libs/libgcrypt.so: undefined reference to `_gcry_mpih_lshift'
    i586-pc-gnu-ld: ./.libs/libgcrypt.so: undefined reference to `_gcry_mpih_add_n'
    i586-pc-gnu-ld: ./.libs/libgcrypt.so: undefined reference to `_gcry_mpih_addmul_1'
    i586-pc-gnu-ld: ./.libs/libgcrypt.so: undefined reference to `_gcry_mpih_submul_1'
    i586-pc-gnu-ld: ./.libs/libgcrypt.so: undefined reference to `_gcry_mpih_mul_1'
    i586-pc-gnu-ld: ./.libs/libgcrypt.so: undefined reference to `_gcry_mpih_sub_n'
    i586-pc-gnu-ld: ./.libs/libgcrypt.so: undefined reference to `_gcry_mpih_rshift'

* gnu/packages/gnupg.scm (libgcrypt)[arguments]: Disable assembly when
cross-compiling.