Add ability to change compiler versions for androideabi

Fix indents; Indent is two spaces; Delete tabs [skip ci]

Added documentation for hash.

Don't crash if pattern not found for sub

64bit Cygwin also doesn't have frexpl(3)

use mrb_str_cat_str() instead of mrb_str_append()

delete mrb_free()-related non-NULL checks

No need to optimize since a program only exits once and errors are rare.
Also the mruby source code doesn't have these kind of checks elsewhere.

The ones in {Time,Random}#initialize are kept because there it actually
matters
since initialization always happens and re-initialization is unlikely.

If the argument is always a string, then mrb_str_cat_str() can be
directly called instead of indirectly by mrb_str_append().

mrb_any_to_s(), mrb_obj_as_string(), mrb_inspect() always return a
string.

(`mrb_rescue_exceptions` is mruby implementation of `rb_rescue2`.)
Closes #2844, closes #2837.

Doc bang modifier

S,A,H gives nil, s,z,a,d gives NULL for nil argument. ref 02474daa

thus "S!" means String|nil type. you have to check nil before
dereferencing the value.  this is added to address #2882 while
keeping code simplicity. besides that current #2882 fix lose
polymorphism provided by mrb_get_args().

Note: arguments of mrb_no_method_error() has changed. You need to replace
3rd and 4th argument (say n, argv) to mrb_ary_new_from_values(mrb, n, argv).

Patching array join

implement llround. some platform (ex: mingw32) doesn't have llround.