mm: remove VM_EXEC requirement for THP eligibility

Commit e6be37b2

 ("mm/huge_memory.c: add missing read-only THP checking
in transparent_hugepage_enabled()") introduced the VM_EXEC requirement,
which is not strictly needed.

lld's default --rosegment option and GNU ld's -z separate-code option
(default on Linux/x86 since binutils 2.31) create a read-only PT_LOAD
segment without the PF_X flag, which should be eligible for THP.

Certain architectures support medium and large code models, where .lrodata
may be placed in a separate read-only PT_LOAD segment, which should be
eligible for THP as well.

Link: https://lkml.kernel.org/r/20231220054123.1266001-1-maskray@google.com
Signed-off-by: Fangrui Song <maskray@google.com>
Acked-by: Yang Shi <shy828301@gmail.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Song Liu <songliubraving@fb.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>