gdb: fix unittests/gmp-utils-selftests.c build on solaris

gdb: fix unittests/gmp-utils-selftests.c build on solaris

When building on solaris (gcc farm machine gcc211), I get:

      CXX    unittests/gmp-utils-selftests.o
    /export/home/simark/src/binutils-gdb/gdb/unittests/gmp-utils-selftests.c: In function 'void selftests::gdb_mpz_read_all_from_small()'  :
    /export/home/simark/src/binutils-gdb/gdb/unittests/gmp-utils-selftests.c:128:43: error: call of overloaded 'pow(int, int)'   is ambiguous
       LONGEST l_min = -pow (2, buf_len * 8 - 1);
                                               ^
    In file included from /opt/csw/lib/gcc/sparc-sun-solaris2.10/5.5.0/include-fixed/math.h:22:0,
                     from ../gnulib/import/math.h:27,
                     from /export/home/simark/src/binutils-gdb/gdb/unittests/gmp-utils-selftests.c:23:
    /opt/csw/lib/gcc/sparc-sun-solaris2.10/5.5.0/include-fixed/iso/math_iso.h:210:21: note: candidate: long double std::pow(long double, long double)
      inline long double pow(long double __X, long double __Y) { return
                         ^
    /opt/csw/lib/gcc/sparc-sun-solaris2.10/5.5.0/include-fixed/iso/math_iso.h:170:15: note: candidate: float std::pow(float, float)
      inline float pow(float __X, float __Y) { return __powf(__X, __Y); }
                   ^
    /opt/csw/lib/gcc/sparc-sun-solaris2.10/5.5.0/include-fixed/iso/math_iso.h:71:15: note: candidate: double std::pow(double, double)
     extern double pow __P((double, double));
                   ^

The "pow" function overloads only exist for float-like types, and the
compiler doesn't know which one we want.  Change "2" for "2.0", which
makes the compiler choose one alternative (the double one, I believe).

gdb/ChangeLog:

	* unittests/gmp-utils-selftests.c (gdb_mpz_read_all_from_small):
	Pass 2.0 to pow.
	(gdb_mpz_write_all_from_small): Likewise.

Change-Id: Ied2ae0f01494430244a7c94f8a38b07d819f4213