Enable common subexpression optimization in GCC.

Enable common subexpression optimization in GCC.

* lib/unitypes.in.h (_UC_ATTRIBUTE_CONST, _UC_ATTRIBUTE_PURE): New
macros.
* lib/unicase.in.h (uc_toupper, uc_tolower, uc_totitle): Declare with
GCC attribute 'const'.
(uc_locale_language): Declare with GCC attribute 'pure'.
* lib/unicase/caseprop.h (uc_is_cased, uc_is_case_ignorable): Declare
with GCC attribute 'const'.
* lib/unictype.in.h (uc_is_general_category_withtable,
uc_combining_class, uc_combining_class_name,
uc_combining_class_long_name, uc_bidi_class_name,
uc_bidi_category_name, uc_bidi_class_long_name, uc_bidi_class,
uc_bidi_category, uc_is_bidi_class, uc_is_bidi_category,
uc_decimal_value, uc_digit_value, uc_numeric_value,
uc_joining_type_name, uc_joining_type_long_name, uc_joining_type,
uc_joining_group_name, uc_joining_group, uc_is_property_*, uc_script,
uc_block, uc_is_c_whitespace, uc_is_java_whitespace,
uc_c_ident_category, uc_java_ident_category, uc_is_alnum, uc_is_alpha,
uc_is_cntrl, uc_is_digit, uc_is_graph, uc_is_lower, uc_is_print,
uc_is_punct, uc_is_space, uc_is_upper, uc_is_xdigit, uc_is_blank):
Declare with GCC attribute 'const'.
(uc_general_category_name, uc_general_category_long_name,
uc_general_category_byname, uc_general_category,
uc_is_general_category, uc_combining_class_byname,
uc_bidi_class_byname, uc_bidi_category_byname, uc_joining_type_byname,
uc_joining_group_byname, uc_script_byname, uc_is_script, uc_is_block):
Declare with GCC attribute 'pure'.
* lib/unigbrk.in.h (uc_graphemeclusterbreak_property,
uc_is_grapheme_break, u8_grapheme_next, u16_grapheme_next,
u32_grapheme_next, u8_grapheme_prev, u16_grapheme_prev,
u32_grapheme_prev, lib/uniname.in.h (unicode_name_character): Declare
with GCC attribute 'pure'.
* lib/uninorm.in.h (uc_composition): Declare with GCC attribute
'const'.
(uninorm_decomposing_form): Declare with GCC attribute 'pure'.
* lib/unistr.in.h (): Declare with GCC attribute 'const'.
(u8_check, u16_check, u32_check, u8_mblen, u16_mblen, u32_mblen,
u8_cmp, u16_cmp, u32_cmp, u8_cmp2, u16_cmp2, u32_cmp2, u8_chr, u16_chr,
u32_chr, u8_mbsnlen, u16_mbsnlen, u32_mbsnlen, u8_strmblen,
u16_strmblen, u32_strmblen, u8_strlen, u16_strlen, u32_strlen,
u8_strnlen, u16_strnlen, u32_strnlen, u8_strcmp_gnu, u8_strcmp,
u16_strcmp, u32_strcmp, u8_strncmp, u16_strncmp, u32_strncmp,
u8_strchr, u16_strchr, u32_strchr, u8_strrchr, u16_strrchr,
u32_strrchr, u8_strcspn, u16_strcspn, u32_strcspn, u8_strspn,
u16_strspn, u32_strspn, u8_strpbrk, u16_strpbrk, u32_strpbrk,
u8_strstr, u16_strstr, u32_strstr, u8_startswith, u16_startswith,
u32_startswith, u8_endswith, u16_endswith, u32_endswith): Declare with
GCC attribute 'pure'.
* lib/uniwbrk.in.h (uc_wordbreak_property): Declare with GCC attribute
'const'.
* lib/uniwidth.in.h (uc_width): Simplify declaration.
(u8_width, u16_width, u32_width, u8_strwidth, u16_strwidth,
u32_strwidth): Declare with GCC attribute 'pure'.