Rollup merge of #144150 - Gelbpunkt:globalmerge, r=Mark-Simulacrum

tests: assembly: cstring-merging: Disable GlobalMerge pass

The test relies on LLVM not merging all the globals into one and would currently otherwise fail on powerpc64le.

See https://github.com/llvm/llvm-project/blob/release/20.x/llvm/lib/CodeGen/GlobalMerge.cpp and here's the assembly generated prior to disabling the pass:

<details>

<summary>Expand me</summary>

```asm
	.abiversion 2
	.file	"cstring_merging.5aa81ea7b99b31fe-cgu.0"
	.section	.text.cstr,"ax",``@progbits``
	.globl	cstr
	.p2align	4
	.type	cstr,``@function``
cstr:
.Lfunc_begin0:
	.cfi_startproc
.Lfunc_gep0:
	addis 2, 12, .TOC.-.Lfunc_gep0@ha
	addi 2, 2, .TOC.-.Lfunc_gep0@l
.Lfunc_lep0:
	.localentry	cstr, .Lfunc_lep0-.Lfunc_gep0
	addis 3, 2, .L_MergedGlobals@toc@ha
	li 4, 4
	addi 3, 3, .L_MergedGlobals@toc@l
	addi 3, 3, 4
	blr
	.long	0
	.quad	0
.Lfunc_end0:
	.size	cstr, .Lfunc_end0-.Lfunc_begin0
	.cfi_endproc

	.section	.text.manual_cstr,"ax",``@progbits``
	.globl	manual_cstr
	.p2align	4
	.type	manual_cstr,``@function``
manual_cstr:
.Lfunc_begin1:
	.cfi_startproc
.Lfunc_gep1:
	addis 2, 12, .TOC.-.Lfunc_gep1@ha
	addi 2, 2, .TOC.-.Lfunc_gep1@l
.Lfunc_lep1:
	.localentry	manual_cstr, .Lfunc_lep1-.Lfunc_gep1
	addis 3, 2, .L_MergedGlobals@toc@ha
	li 4, 4
	addi 3, 3, .L_MergedGlobals@toc@l
	addi 3, 3, 8
	blr
	.long	0
	.quad	0
.Lfunc_end1:
	.size	manual_cstr, .Lfunc_end1-.Lfunc_begin1
	.cfi_endproc

	.type	CSTR,``@object``
	.section	.data.rel.ro.CSTR,"aw",``@progbits``
	.globl	CSTR
	.p2align	3, 0x0
CSTR:
	.quad	.L_MergedGlobals
	.size	CSTR, 8

	.type	.L_MergedGlobals,``@object``
	.section	.rodata..L_MergedGlobals,"a",``@progbits``
.L_MergedGlobals:
	.asciz	"foo"
	.asciz	"bar"
	.asciz	"baz"
	.size	.L_MergedGlobals, 12

.set .Lanon.a643e9a6bba67b7953be2b5f96e0e802.0, .L_MergedGlobals
	.size	.Lanon.a643e9a6bba67b7953be2b5f96e0e802.0, 4
.set .Lanon.a643e9a6bba67b7953be2b5f96e0e802.1, .L_MergedGlobals+4
	.size	.Lanon.a643e9a6bba67b7953be2b5f96e0e802.1, 4
.set .Lanon.a643e9a6bba67b7953be2b5f96e0e802.2, .L_MergedGlobals+8
	.size	.Lanon.a643e9a6bba67b7953be2b5f96e0e802.2, 4
	.ident	"rustc version 1.90.0-dev"
	.section	".note.GNU-stack","",``@progbits``
```

</details>

Author: GuillaumeGomez