riscv: vdso: Exclude .rodata from the PT_DYNAMIC segment

.rodata is implicitly included in the PT_DYNAMIC segment due to
inheriting the segment of the preceding .dynamic section (in both GNU ld
and LLD).  When the .rodata section's size is not a multiple of 16
bytes on riscv64, llvm-readelf will report a "PT_DYNAMIC dynamic table
is invalid" warning.  Note: in the presence of the .dynamic section, GNU
readelf and llvm-readelf's -d option decodes the dynamic section using
the section.

This issue arose after commit 8f8c1ff
("riscv: vdso.lds.S: remove hardcoded 0x800 .text start addr"), which
placed .rodata directly after .dynamic by removing .eh_frame.

This patch resolves the implicit inclusion into PT_DYNAMIC by explicitly
specifying the :text output section phdr.

Reported-by: Nathan Chancellor <nathan@kernel.org>
Closes: ClangBuiltLinux#2093
Signed-off-by: Fangrui Song <i@maskray.me>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20250602-riscv-vdso-v1-1-0620cf63cff0@maskray.me
Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>

Author: MaskRay

arch/riscv/kernel/vdso/vdso.lds.S

@@ -30,7 +30,7 @@ SECTIONS
 		*(.data .data.* .gnu.linkonce.d.*)
 		*(.dynbss)
 		*(.bss .bss.* .gnu.linkonce.b.*)
-	}
+	}						:text
 
 	.note		: { *(.note.*) }		:text	:note