x86: Make stringop_algs::stringop_strategy ctor constexpr [PR100246]

x86: Make stringop_algs::stringop_strategy ctor constexpr [PR100246]

> Several older compilers fail to build modern GCC because of missing
> or incomplete C++11 support.
>
>       * config/i386/i386.h (struct stringop_algs): Define a CTOR for
>       this type.

Unfortunately, as mentioned in my
https://gcc.gnu.org/pipermail/gcc-patches/2021-November/583289.html
mail, without the new dyninit pass this causes dynamic initialization of
many variables, 6.5KB _GLOBAL__sub_I_* on x86_64 and 12.5KB on i686.

The following patch makes the ctor constexpr so that already the FE
is able to statically initialize all those.

I have tested on godbolt a reduced testcase without a constructor,
with constructor and with constexpr constructor.
clang before 3.3 is unhappy about all the 3 cases, clang 3.3 and 3.4
is ok with ctor and ctor with constexpr and optimizes it into static
initialization, clang 3.5+ is ok with all 3 versions and optimizes,
gcc 4.8 and 5+ is ok with all 3 versions and no ctor and ctor with constexpr
is optimized, gcc 4.9 is unhappy about the no ctor case and happy with the
other two.

2021-11-05  Jakub Jelinek  <jakub@redhat.com>

	PR bootstrap/100246
	* config/i386/i386.h
	(stringop_algs::stringop_strategy::stringop_strategy): Make the ctor
	constexpr.