make ease to use foreign attributes about nvc HOT 3 CLOSED

$ cat ext.c
#include <stdint.h>

int32_t ext_add_int32_t(int32_t l, int32_t r) { return l + r; }
int32_t ext_sub_int32_t(int32_t l, int32_t r) { return l - r; }
int32_t ext_mul_int32_t(int32_t l, int32_t r) { return l * r; }
int32_t ext_div_int32_t(int32_t l, int32_t r) { return l / r; }

double ext_add_double(double l, double r) { return l + r; }
double ext_sub_double(double l, double r) { return l - r; }
double ext_mul_double(double l, double r) { return l * r; }
double ext_div_double(double l, double r) { return l / r; }

$ cat ext.vhd
package ext is
        pure function ext_add(l : integer; r : integer) return integer;
        pure function ext_sub(l : integer; r : integer) return integer;
        pure function ext_mul(l : integer; r : integer) return integer;
        pure function ext_div(l : integer; r : integer) return integer;
        attribute foreign of ext_add : function is "ext_add_int32_t";
        attribute foreign of ext_sub : function is "ext_sub_int32_t";
        attribute foreign of ext_mul : function is "ext_mul_int32_t";
        attribute foreign of ext_div : function is "ext_div_int32_t";
end package ext;

use work.ext.all;

entity test is
begin
end entity test;

architecture a of test is
begin
        process
                constant w : integer range 1 to integer'high := 4;
        begin
                for l in 0 to 2**w-1 loop
                        for r in 0 to 2**w-1 loop
                                assert (ext_add(l, r) = l + r);
                                assert (ext_sub(l, r) = l - r);
                                assert (ext_mul(l, r) = l * r);
                                if (r /= 0) then
                                        assert (ext_div(l, r) = l / r);
                                end if;
                        end loop;
                end loop;
                wait;
        end process;
end architecture a;

$ NVC_LIBPATH=./lib/std:./lib/ieee:./lib/nvc NVC_CYG_LIB=./src ./src/nvc.exe -a ext.vhd

$ NVC_LIBPATH=./lib/std:./lib/ieee:./lib/nvc NVC_CYG_LIB=./src NVC_FOREIGN_OBJ=./ext.o ./src/nvc.exe -e --native test
/usr/bin/opt -O2 -o /home/hiyuh/git-repos/nvc/work/_WORK.TEST.final.bc /home/hiyuh/git-repos/nvc/work/_WORK.TEST.elab.bc
/usr/bin/llc -relocation-model=pic /home/hiyuh/git-repos/nvc/work/_WORK.TEST.final.bc
/usr/bin/gcc -shared -Wl,--export-all-symbols -Wl,--out-implib=/home/hiyuh/git-repos/nvc/work/_WORK.TEST.elab.a -o /home/hiyuh/git-repos/nvc/work/_WORK.TEST.final.so /home/hiyuh/git-repos/nvc/work/_WORK.TEST.final.s ./ext.o -L./src -lnvcimp

$ NVC_LIBPATH=./lib/std:./lib/ieee:./lib/nvc NVC_CYG_LIB=./src ./src/nvc.exe -r test
[no assertion faulure]