supersize: Fix --toolprefix="" not being followed

tools/binary_size/libsupersize/testdata/Console.golden

 ********************************************************************************
 Entering interactive Python shell. Quick reference:
 
 SizeInfo: Cluster, metadata, section_sizes, symbols
 Symbol: FlagsString, IsBss, IsGeneratedByToolchain, IsGroup, address, aliases, end_address, flags, full_name, generated_source, is_anonymous, name, num_aliases, object_path, padding, pss, pss_without_padding, section, section_name, size, size_without_padding, source_path
 
 SymbolGroup (extends Symbol): Cluster, CountUniqueSymbols, Filter, GroupBy, GroupByNamespace, GroupByPath, GroupBySectionName, Inverted, IterLeafSymbols, IterUniqueSymbols, Sorted, SortedByAddress, SortedByCount, SortedByName, WhereAddressInRange, WhereBiggerThan, WhereFullNameMatches, WhereGeneratedByToolchain, WhereHasAnyAttribution, WhereHasPath, WhereInSection, WhereMatches, WhereNameMatches, WhereObjectPathMatches, WherePathMatches, WhereSourceIsGenerated, WhereSourcePathMatches, is_sorted
 
 SymbolDiff (extends SymbolGroup): IsAdded, IsRemoved, IsSimilar, WhereNotUnchanged, added_count, changed_count, removed_count, unchanged_count
 
@@ skipping 29 matching lines @@
 Print(Diff(size_info1, size_info2), to_file="output.txt")
 
 # View per-component breakdowns, then drill into the last entry.
 c = canned_queries.CategorizeByChromeComponent()
 Print(c)
 Print(c[-1].GroupByPath(depth=2).Sorted())
 
 # For even more inspiration, look at canned_queries.py
 # (and feel free to add your own!).
 Metadata:
-    elf_arch=ARM
+    elf_arch=arm
     elf_build_id=WhatAnAmazingBuildId
     elf_file_name=elf
     elf_mtime={redacted}
     git_revision=abc123
     gn_args=var1=true var2="foo"
     map_file_name=../test.map
 
 Section Sizes (Total=43,785,380 bytes):
     .bss: 1,300,456 bytes (not included in totals)
     .data: 101,768 bytes (0.2%)
@@ skipping 96 matching lines @@
 43)  43785380 (98.8%) b@0x0        131072  third_party/fft_fixed.cc
              ff_cos_131072_fixed
 44)  43785380 (98.8%) b@0x0        131072  third_party/fft_float.cc
              ff_cos_65536
 45)  43785380 (98.8%) b@0x2dffda0  28      third_party/icu/ucnv_ext.c
              g_chrome_content_browser_client
 46)  43785380 (98.8%) b@0x2dffe80  200     third_party/icu/ucnv_ext.c
              SaveHistogram::atomic_histogram_pointer
 47)  43785380 (98.8%) b@0x2dffe84  4       third_party/icu/ucnv_ext.c
              g_AnimationFrameTimeHistogram_clazz